Visible light sources--and usually white or near-white light sources--are demanded for illumination throughout the world. Fluorescent, incandescent, and other lamps and bulb have become ubiquitous for illumination. In addition, light-emitting diodes, or LEDS, have become very common for single or limited wavelength forms of illumination such as laser pointers, indicator lamps and so on.
Recently, considerable attention has been given to the potential for using LEDs as a white light source because the power consumption of LEDs is, generally, lower than that of fluorescent lamps or bulb lamps. However, it has been difficult to use such LEDs as sources for white light due to the unavailability highly efficient blue LEDs, Recent progress in group III-nitride semiconductor systems has enabled the development of the InGaN/GaN DH (Double Heterojunction) structure, which does permit the high quantum efficiency emission of blue light. As a result, blue LEDs are now commercially available.
Obtaining white light from blue LEDs has been problematic. One effective approach to obtain white light is to mix blue and yellow light. Referring first to FIGS. 1 and 2, there is first shown in schematic form the emission of white light using a prior art blue LED combined with a yellow phosphor (S. Nakamura, SPIE, Vol. 3002, pp26-35, 1997.) In the prior art arrangement shown in FIG. 2, YAG (yttrium aluminum garnet) phosphor 1 is formed on the top of a blue LED 2 having an InGaN/GaN DH structure. In this structure, current is injected from the lead frame 3 with a reflector cup 4 to the LED chip 2 through a conductor 5. Then the current flows from the LED chip 2 to the lead frame 6. The LED chip 2 is activated by the injected current and emits blue light. The YAG phosphor 1 is excited by the blue light from the LED chip 2 and emits yellow fluorescence. The mixture of the blue emission from the LED chip 2 and the yellow emission from the phosphor 1 results in a white emission. In order to improve the external emission efficiency of the white light, an epoxy lens 7 is used. Thus white light is obtained by this structure.
However, to obtain a white light source panel having a large area size, it is necessary to use large number of LEDs. For the prior art design shown in FIGS. 1 and 2, the driving circuit becomes complicated and the fabrication cost becomes high. Moreover, as shown in FIG. 3, the luminescence pattern of the panel becomes dot-like when a large number of conventional LEDs are gathered into a single panel. This results from the "point source" nature of conventional LED's. Therefore, it is difficult to use a large number of prior art LEDs to provide a white light panel having uniformly distributed emission power. Further, while a thin lighting panel is preferred, in the art shown in FIGS. 1-3 the thickness of the panel becomes large because the thickness of the panel must be larger than the height of the LEDs. These problems must be solved to obtain a thin white light source panel having a large area with uniform white light emission.